Releasing a mod simply for a portrait seems a little silly, doesn't it?
Still, I'd say it depends on what
you want. Of course including a ToB portion is more work and takes longer to finish, but it grants the player a complete version of the NPC. New things can always be added later, as well. Yasraena, for example, was released with full SoA+ToB content.
Personally, I would wait with releasing any serious projects until the ToB portion is finished in addition to SoA. In my case, that would be Lockheart and Aerie Relationship. Any other NPC mod I toy with nowadays, if I ever manage to finish a SoA version, would probably be released as SoA first, and ToB later.
I wouldn't mind waiting for Halbo SoA+ToB.